Overheating Tin Top

[Guest timswait]

Sorry for the non- kit car post, but while I'm still re-building the Hood my tin top is now having issues, so any help getting this sorted will give me time to get back to the build and indirectly help get it finished, honest

I've got an Audi A4 1.9TDi, 2002 with 150k miles on it, and recently it's started having over heating issues. This first manifested itself completely out of the blue while driving up a prolonged hill, just as I got to the top the gauge suddenly shot into the red and the coolant warning message came on. I stopped, let it cool (it had boiled out some coolant), so I topped it up and tried to continue, however the next long hill I got to the same thing happened, the gauge was reading spot on 90 until very suddenly shooting over the warning coming on and steam from the reservoir. Again let it cool, topped it up and drove home very carefully and slowly. Tried plugging in a laptop to do some diagnostics, no fault codes shown so tried some test runs reading outputs. I found a readout for Coolant temperature from the ECU so drove around comparing that to the gauge. All the time around town they read identical, at temperature the gauge was reading 90 and the ECU read 88-91. However I tried a test run up a long hill in the peaks (the same one that caused the first boiling over problem) and then they started to diverge, the gauge stayed rock steady on 90 but the ECU got up to 118(!) by the top of the hill. It didn't boil over or trigger the warning that time, but I assume if the hill had carried on any further it would have, the ECU readout was still going up. So what I suspect is the ECU is reading the coolant temp in the block but the sensor for the gauge is somewhere else, in a pipe probably.. So for some reason the coolant is overheating in the block until it boils, and at that point forces the boiling water out and suddenly the gauge starts reading it, however I don't know why this is happening.

I took it to the garage as I wasn't sure what to try. They suggested it could be a head gasket, so tried using an exhaust gas analyser probe to sniff around the coolant reservoir (cap removed) with the engine running. However this couldn't detect any exhaust fumes and there were no other symptoms, no smoke or steam from exhaust, no bubbles in the reservoir or sign of it pressurising, no sign of oil in the water or water in the oil. We decided the thermostat was the next thing to try. I had actually already changed the thermostat a year ago (unrelated to this issue, the engine wasn't getting upto temperature at the time), so thought it unlikely, but didn't have any better ideas, so they changed it and also changed and flushed the coolant while they were about it. They (and me) were doubtful it would work, they reported the coolant looked pretty clean and the thermostat seemed to open and shut when they tested it (although maybe it was a tad sluggish). However I took it out for another test run with the laptop connected, drove up the same hill as caused the problems before and this time the ECU measured temperature stayed fine, didn't go above 93C. I was chuffed, thinking it was solved!

However, that was a month ago and now I'm noticing that the coolant level is going down I drove around with it with the laptop connected today reading the coolant temperature recorded by the ECU. For a while it seemed OK, not going over 92C even pushing it a bit, but after half an hour it started to rise when going up hills, at one point it got over 100C going up a hill, the gauge was still on 90. So now I think it's probably an intermittent problem, I'm not sure that the work the garage did actually did anything, it was maybe just luck that it seemed solved at the time.

Does anyone have any ideas what would cause these symptoms? I know this is a long post, so if you've scanned through to the end, here's a summary!

-Overheating (according the coolant temp read by the ECU) under sustained heavy load.

-Even though the coolant temp read by the ECU is way too high (up to 118C) the gauge on the dash still reads 90.

-If you really push it hard then it boils over, at which point the dash gauge suddenly jumps into the red and the coolant warning alarm comes on.

-No symptoms that really suggest head gasket.

-Already tried changing the thermostat and flushing and changing the coolant.

Ideas please

[fry61]

You haven't mentioned when/if the radiator fan cuts in.

[Guest timswait]

It has both an electric and a thermo-viscous fan (strange, dunno why two fans of two different types, but it appears OEM). The electric one was running when I opened the bonnet after the boiling over, although the thermo-viscous one did still feel loose. The temperature stays rock steady (according to gauge and ECU) when sat in traffic. Also on the boiling over occasion it was going up a hill in the peaks, so I was doing around 60mph, so had plenty of airflow. I'm a little suspicious of the thermo-viscous fan, but it doesn't really seem to explain it. Also doesn't explain why the two temperature measurements are so different.

[fry61]

Maybe put a temporary over ride switch in circuit to feed the electric fan & use it to see if temp can be kept down; or maybe a warning lamp connected across the fan terminals to show when it cuts in -- or both.

[Guest timswait]

It might be one of the parameters I can read from the ECU, I'll take a look to see if I can watch that as well to see when it's cutting in. I suspect it won't be cutting in because it'll probably be reading the temperature from the same source as the dash gauge, but if that is the case I think that's a symptom not a cause, the dash gauge should be reading it as being hot if it is hot, parts of the system seem to be boiling while other parts are fine. And if 60mph wind through the grill doesn't cool it then I doubt the fan would be able to. My thoughts are the water is not being circulated strongly enough meaning temperature is building in the engine but not making it into the rad. That thinking led me to the water pump but the water pump is driven off the timing belt, and was changed when the timing belt was changed 20k miles ago, so should be OK.

[Guest Gargoil]

Any evidence of where the coolant is going?

Radiators don't last forever, could be blocked or loosing fluid/not pressurising.

[Grim]

is the cooling system pressurising properly? what pressure is it supposed to be? 118°C would be about the boiling over temp on a 13psi system. If there was a slow leak it might pressurise initially but drop down over time, giving you the current symptoms?

I know some modern gauges are not linear - ie they are programmed to show 'normal' at a broad range of temperatures and to only go up in emergencies, so the discrepancy between ecu temp and the gauge may be normal.

[richyb66]

Cracked coolant header tank - I'd almost put money on it.

 

I've had 2 of these, one on a 1.9tdi Audi 80 and the other on my S6. The cracks are usually very small on the underside of the tank so not immediately obvious but the coolant boils out slowly and the cooling system doesn't hold pressure so it boils easily.

 

The tanks are suprisingly cheap fortunately.

Edited June 11, 2014 by richyb66

[Guest Ian & Carole]

 

Cracked coolant header tank - I'd almost put money on it.

 

The tanks are suprisingly cheap fortunately.

 

 

Cheap original manufacturers parts are always a sign that they have an issue with them and therefore offer them at a seriously cheap price, if you asked that question thought hey would deny it.

Even back in the dark ages when I worked at a BMW main dealer some of the regular fails were cheap as chips.

[Guest timswait]

I find it hard to tell where the coolant is going! There's never a puddle underneath the car when I leave it parked and there's no obvious leaks. On the occasions where it's boiled over there has been general dampness and marks around the reservoir. I was assuming it was boiling and forcing it out of the cap, but maybe it is coming from underneath. I was assuming that it was pressurising on the basis that it was getting up to 118C and not erupting in steam, but perhaps it is loosing pressure. Any easy way to tell if it's holding pressure? The cracked expansion tank sounds a good call, thank you for that Richy. I'll see if I can get a good look at it at lunchtime to check it. Any idea where can i get expansion tanks from, is it a dealer only thing? I can find a lot of secondhand on EBay, but I'd prefer to use a new one as if I fit another one the same age then sods law it'll do the same thing again in a year's time!

[richyb66]

The leaking is usually quite subtle so you don't get pooling coolant, it just comes out as steam which you're only likely to spot on a cold day. Feel under the tank, you can sometimes get coolant residue on the tank which is an easy spot.

 

Tank is probably main dealer only but should be under £40 (can't remember what the last one cost me but it wasn't too bad). Part number is most likely 8E0121403 but dealer will confirm from VIn or reg number, get a new cap as well for the extra few quid. Second had not worth bothering with, they could be just as bad as your current on

[knights_templar]

My lad works for an Audi specialist, I related your problem to him this morning, without testing he would assume water pump impeller, they are plastic and can start slipping under load.

 

Was amused by sniffer test, they are inconclusive with an intermittent fault and completely useless on a diesel, despite what it says on the tin. but it looks good

[Guest timswait]

A small crack that allows it to pressurise initially but then starts to seep and loose pressure might fit the symptom..... The pump was fitted before I got the car, so I don't know what was used. The garage that did the work wasn't a dealer, the sticker on the cam cover for the timing belt was a Conti-tech one, so probably not an OEM part.

[Grim]

Tim, They're £14 from euro car parts online, even without the rhocar discount. Got to be worth a try.

[Guest timswait]

There's no obvious crack in the water tank, but there is dampness underneath it. And I'm pretty sure it didn't run to hot on the way into work, so it really shouldn't have boiled out, so it does look like a leak. It's £16 for a genuine Audi one, and it's such an easy job to change I'll do that first, see if that sorts it